Radiation is commonly used in the non-invasive inspection of objects such as luggage, bags, briefcases, and the like, to identify hidden contraband at airports and public buildings. The contraband may include hidden guns, knives, explosive devices and illegal drugs, for example.
FIG. 1 is a front view of an example of an X-ray scanning system 10, referred to as a line scanner, for scanning luggage in airports, for example. The object 12 to be inspected is conveyed through a shielded tunnel 13 between a stationary source of radiation 14, such as X-ray radiation, and a stationary detector array 16, by a conveying system 18. The source 14 is typically a source of X-ray radiation of about 160 KeV to about 450 KeV. The source may be an X-ray tube, for example. The radiation is collimated into a fan beam 20 that intercepts the entire height of the object 12. Windows 21a, 21b are provided in the walls of the tunnel 13 to allow for the passage of radiation to the object 12 from the source 14 and from the object to the detector array 16. The detector array 16 may also be provided within the shielded tunnel 13, in which case only one window 21a would be required. The conveyor system 18 may comprise a mechanically driven belt comprising material that causes low attenuation of the radiation. The conveyor system 18 can also comprise mechanically driven rollers, with gaps in the rollers to allow for the passage of the radiation. Shielding walls 22 surround the source 14, the detector 16 and a portion of the conveying system 18. Openings (not shown) are provided in the shielding walls 22 for the object to be conveyed into and out of the scanning system 10 by the conveying system 18. Such systems are generally large. In another example, the object may be stationary and the radiation source and detector may be moved past the object by a suitable conveying system.
Radiation transmitted through the object 12 is attenuated to varying degrees by the object and its contents. The attenuation of the radiation is a function of the amount (thickness) and atomic composition of the materials through which the radiation beam passes. The attenuated radiation is detected and radiographic images of the contents of the object 12 are generated for inspection. The images show the shape, size and varying densities of the contents.
Standard cargo containers are typically 20-50 feet long (6.1-15.2 meters), 8 feet high (2.4 meters) and 6-9 feet wide (1.8-2.7 meters). Air cargo containers, which are used to contain plural pieces of luggage or other objects to be stored in the body of an airplane, may range in size from about 35×21×21 inches (0.89×0.53×0.53 meters) up to about 240×96×118 inches (6.1×2.4×3.0 meters). Large collections of objects, such as many pieces of luggage, may also be supported on a pallet. Pallets, which may have supporting sidewalls, may be of comparable sizes as cargo containers. The term “cargo conveyance” is used herein to refer to both cargo containers and pallets. The low energies used in typical X-ray luggage and bag scanners, described above (160 KeV-450 KeV), are too low to penetrate through the much larger cargo containers or collections of objects. In addition, many such systems are too slow to economically inspect larger objects, such as cargo containers.
To penetrate larger cargo containers, X-ray radiation of at least about 1 MeV range is generally required. Linear accelerators may be used to generate X-ray radiation in the MeV range. The actual size may depend on the frequency band of the accelerator and the amount of shielding, which is dependent on the energy and flux of the generated radiation, as well as the expected locations of people and the amount of time those people may be exposed to radiation. Typical X-ray scanning system used to screen cargo containers have lengths ranging from about 1 meter to about 5 meters for 3-4 MeV system, and up to about 3.3 meters for a 5-15 MeV system. In addition, the intensity of the radiation is greatest in a forward direction, along the longitudinal axis of the electron beam. The uniformity of the emitted radiation decreases as the angle from the forward direction is increased. To maintain dose rate uniformity, at peak X-ray energy of about 9 MeV, for example, a narrow beam having a total arc of up to about 30 degrees tends to be used. At a peak energy of about 3 MeV, a beam having an arc up to about 65-70 degrees may be used. The smaller the arc, the farther the source must be in order to intercept the entire object. The length of the high energy X-ray sources and the beam arc tend to make higher energy X-ray scanning systems large. At these high energies, more shielding is also generally required.
It has been found to be difficult to distinguish nuclear devices and nuclear materials from other dense or thick items that may be contained within the object by standard X-ray scanning. The information that may be derived about the material type of the contents of objects by X-ray scanning may be enhanced by the use of radiation beams in the MeV energy range, with two or more different energy spectra that interact differently with the material contents of the object. For example, the attenuation of a 6 MeV X-ray radiation beam by the contents of the object will be different from the attenuation a 9 MeV X-ray radiation beam by the same contents, due to the differing effects of Compton Scattering and induced pair production on the different energy beams. A ratio of the attenuations at the two X-ray energies may be indicative of the atomic numbers of the material through which the radiation beam passes, as described in U.S. Pat. No. 5,524,133, for example, which is incorporated by reference herein. More sophisticated dual energy analysis techniques are described in U.S. Pat. No. 7,257,188, for example, which is assigned to the assignee of the present invention and incorporated by reference herein. Ratios of high and low energy attenuations may also be plotted against object thickness to facilitate material identification, as described in “Dual Energy X-ray radiography for automatic high-Z material detection,” G. Chen et al, NIM (B), Volume 261 (2007), pp. 356-359, which is also incorporated by reference herein.